1. Field of the Invention
This invention relates to a digital video effects system for the manipulation of pictures represented by video signals. In particular it relates to the provision of lighting effects in such a system.
2. Description of the Prior Art
The art of manipulating pictures represented by digital video signals is well established. In essence the manipulation is accomplished by: digitising an analogue video signal by sampling it and converting each sample into a binary word of, for example, 8 or 10 bits representing that sample; storing fields or frames of the digitised signal in memory; and controlling either the reading from or the writing to the memory so as to produce from each field or frame a picture that differs from that represented by the input video signal in that at least one geometrical parameter thereof is changed. Such geometrical parameter may, for example, comprise the location of the picture along one or more of up to three axes and/or the angular position of the picture about one or more axes. Other such parameters may comprise the size of the picture (in the horizontal and/or vertical direction thereof), the extent of shearing of the picture, and the perspective of the picture.
FIG. 1 of the accompanying drawings shows in simplified block diagram form the general overview of a typical digital video effects system for effecting such manipulation of a picture. The general kind of apparatus now to be described with reference to FIG. 1 has been embodied in a variety of known propriety items of digital video effects equipment, and the operation and construction thereof is well known to those skilled in the art. The digital video effects system comprises a digital video effects unit, which is designated 10 in FIG. 1, and a control unit 24.
A video signal V.sub.1 representing a picture P.sub.1 that is to be manipulated is input into the digital video effects unit 10 at 11. In this prior art digital video effects unit, manipulation of the input picture P.sub.1 is performed by controlling the read addresses to the memory 14, although write side address mapping is also known. This control of the read side addresses is effected by the address generator 20.
As the mapping process may involve compression of the picture, and, in the absence of corrective measures, compression of the picture can give rise to aliasing which will degrade the quality of the output image, a filter 12 is provided to compensate for the effects of compression, A filter controller 18 determines local scaling factors representative of the amount of compression for localised areas of the image, these local scaling factors being used to control the filter 12 to apply appropriate amounts of filtering to respective areas of the image.
A pixel interpolator 14 can be provided to enable output pixel values to be computed where the address generator 20 does not generate a one-to-one mapping between a storage location in the memory 13 and the output pixels.
A synchronisation delay 15 allows the data output from the memory 13 to be aligned with frame synchronisation information. A digital linear keyer 16 allows the data output from the memory 13 (which is representative of foreground information) to be keyed into the background (represented by a signal B input to keyer 16) for forming the output video V.sub.2 of the output picture P.sub.2. A key processor 22 controls the operation of the digital linear keyer 16. The digital video effects unit 10 is under the control of the control unit 24 which can, but need not, be implemented as a conventional personal computer or a computer workstation with appropriate control software.
A known example of a digital video effects system having the above architecture can perform 3D linear manipulations of high definition video with a very high video output quality. However, the known system is limited to linear manipulations of video, being unsuitable for providing video texture mapping, or free form modeling onto a non-linear surface.
"Video texture mapping" is a term which derives from computer graphics where it is used to describe the mapping of an image onto a 3D surface. Originally this technique was developed for simulating surface texture, an appropriate image being mapped to give the desired effect. However, it has also been used for mapping other images to create other effects. See, for example, the colour plates in the book "Computer Graphics, Principles and Practice", Second Edition by Messrs Foley, Van Dam, Feiner and Hughes, published by Addison Wesley in 1990. In computer graphics applications, however, such video texture mapping is rarely, if ever, performed in real time.
In addition to providing video texture mapping, it is often desirable to provide lighting effects to enhance realism. A number of different techniques for providing lighting effects are known. Examples of techniques For providing lighting effects, including a technique known as the Phong technique, are described in Chapter 16 of the aforementioned book by Messrs Foley, Van Dam, Feiner and Hughes. The provision of lifting effects is conventionally one of the last stages in processing a graphics or video image. Such techniques can be implemented in applications employing ray tracing techniques. Although lighting models based on ray tracing techniques can produce outstanding results, they are extremely calculation intensive. To realise a real time lighting model based on ray tracing techniques would require a large amount of hardware.